Microwave level gauge, or radar level gauge (RLG), systems are in wide use for determining the filling level of a product contained in a tank. Radar level gauging may for example be performed by means of non-contact measurement, whereby electromagnetic signals are radiated towards the product contained in the tank.
The transmitted electromagnetic signals are reflected at the surface of the product, and the reflected signals are received by a receiver or transceiver comprised in the radar level gauge system. Based on the transmitted and reflected signals, the distance to the surface of the product can be determined. Determination of the distance can for example be based on time of flight of the received reflected signals.
Radar level gauge systems are used in marine applications for filling level monitoring and custody transfer control of liquid petroleum products. As an example, natural gas comprising predominantly methane and ethane is often transported at low temperature in liquid form in large cooled tanks arranged to maintain a temperature below the boiling point of the product. Heavier hydrocarbons such as butane and propane, also referred to as liquid petroleum gas (LPG) can be transported in liquid form in essentially spherical pressure tanks configured to withstand the elevated pressure commonly applied to liquefy LPG.
Due to the relatively low reflectivity of liquid natural gas (LNG) and LPG, a signal transmitted by a free radiating antenna may result in a reflected signal which is too small to detect. This may occur for example if the distance from the transceiver to the product is too large or if the surface of the product is disturbed so that the signal is scattered when reflecting at the surface.
In order to facilitate radar level gauge measurements also in large tanks where the distance between the antenna and the surface may be relatively large, typically on the order of several tens of meters, and under difficult measurement conditions, it is known to use a so called still pipe.
Use of a still pipe in a radar level gauge for measuring the level of LNG is for example disclosed by WO2007/049966. The still pipe is in fluid communication with the product in the tank and the filling level can be determined by determining the level of the product within the still pipe. By transmitting the signal within a still pipe, the energy of the transmitted electromagnetic signal is focused within the pipe to a small portion of the surface. Thereby, loss of signal power is reduced and measurement over larger distances and under more difficult measurement conditions is facilitated.
Furthermore, in marine applications, high sea states will cause a movement of the liquid in the tank which in turn results in a considerable mechanical force being applied on structures located in the tank. A still pipe arranged in a tank thus requires a support structure able to withstand considerable force.
Additionally, still pipes require a large amount of material which makes them costly, and they may also be complicated to install in a large tank.